The Lastwall Sentry archetype has a prerequisite of being a "member of the Knights of Lastwall." I don't see a background or anything that mechanically would make a character be a knight (the closest thing I know of is Lastwall Survivor) so I was wondering if there is a mechanical way to make a character a Lastwall knight or if the prereq for the archetype is just something you would include in the characters backstory.
There isn't. You can only become a Knight of Lastwall by your GM letting it happen on the story. And because it is uncommon, you need their permission to write it into your backstory. (Unless I misunderstood how "Access" works. I don't actually have the book to check.)
The access condition for Lastwall Sentry is 'you are from the Eye of Dread region' - being part of the knights is a prerequisite that you have to meet even once you have access, unless waived by the GM.
I'm not sure whether putting 'Knight of Lastwall' in your backstory generally counts or not (though it's not like there is a special mechanic for being a member of an organization), but that part doesn't interact with the rarity/access system.
